Fuel cells convert a fuel and an oxidizing agent, which are locally separated from one another at two electrodes, into electricity, heat and water. Hydrogen or a hydrogen-rich gas can be used as the fuel, oxygen or air as the oxidizing agent. The process of energy conversion in the fuel cell is characterized by a particularly high efficiency. The compact design, power density, and high efficiency of polymer electrolyte membrane fuel cells (PEM fuel cells) make them suitable for use as energy converters, and for these reasons PEM fuel cells in combination with electric motors are gaining growing importance as an alternative to conventional combustion engines.
The hydrogen/oxygen type fuel cell relies on anodic and cathodic reactions which lead to the generation and flow of electrons and electrical energy as a useful power source for many applications. The anodic and cathodic reactions in a hydrogen/oxygen fuel cell may be represented as follows:H2→2H++2e−  (Anode)½O2+2e−→H2O   (Cathode)
Each PEM fuel cell unit contains a membrane electrode assembly positioned between bipolar plates, also known as separator plates, which serve to supply gas and conduct electricity. A membrane electrode assembly (MEA) consists of a polymer electrolyte membrane, both sides of which are provided with reaction layers, the electrodes. One of the reaction layers takes the form of an anode for oxidizing hydrogen and the second reaction layer that of a cathode for reducing oxygen. Gas distribution layers made from carbon fiber paper or carbon fiber fabric or cloth, which allow good access of the reaction gases to the electrodes and good conduction of the electrical current from the cell, are attached to the electrodes. The anode and cathode contain electrocatalysts, which provide catalytic support to the particular reaction (oxidation of hydrogen and reduction of oxygen respectively). The metals in the platinum group of the periodic system of elements are preferably used as catalytically active components. Support catalysts are used in which the catalytically active platinum group metals have been applied in highly dispersed form to the surface of a conductive support material. The average crystallite size of the platinum group metals is between around 1 and 10 nm. Fine-particle carbon blacks have proven to be effective as support materials. The polymer electrolyte membrane consists of proton conducting polymer materials. These materials are also referred to below as ionomers. A tetrafluroethylene-flurovinyl ether copolymer with acid functions, particularly sulfuric acid groups, is preferably used. A material of this type is sold under the trade name Nafion® by E.I. DuPont, for example. Other ionomer materials, particularly fluorine-free examples such as sulfonated polyether ketones or aryl ketones or polybenzimidazoles, can also be used, however.
Fuel cells have been pursued as a source of power for transportation because of their high energy efficiency (unmatched by heat engine cycles), their potential for fuel flexibility, and their extremely low emissions. Fuel cells have potential for stationary and vehicular power applications; however, the commercial viability of fuel cells for power generation in stationary and transportation applications depends upon solving a number of manufacturing, cost, and durability problems.
One of the most important problems is the cost of the proton exchange catalyst for the fuel cell. The most efficient catalysts for low temperature fuel cells are noble metals, such as platinum, which are very expensive. Some have estimated that the total cost of such catalysts is approximately 80% of the total cost of manufacturing a low-temperature fuel cell.
In a typical process, an amount of a desired noble metal catalyst of from about 0.5-4 mg/cm2 is applied to a fuel cell electrode in the form of an ink, or using complex chemical procedures. Such methods require the application of a relatively large load of noble metal catalyst in order to produce a fuel cell electrode with the desired level of electrocatalytic activity, particularly for low temperature applications. The expense of such catalysts makes it imperative to reduce the amount, or loading, of catalyst required for the fuel cell. This requires an efficient method for applying the catalyst.
In recent years, a number of deposition methods, including rolling/spraying, solution casting/hot pressing, and electrochemical catalyzation, have been developed for the production of Pt catalyst layers for PEM fuel cells.
In the case of hydrogen/oxygen fuel cells, some improvements in catalyst application methods have been directed towards reducing the amount of costly platinum catalyst in formulations. Development of compositions, for example, was achieved by combining solubilized perfluorosulfonate ionomer (Nafion®), support catalyst (Pt on carbon), glycerol and water. This led to the use of low platinum loading electrodes. The following publications teach some of these methods for hydrogen/oxygen fuel cells: U.S. Pat. No. 5,234,777 to Wilson; M. S. Wilson, et al, J. App. Electrochem., 22 (1992) 1-7; C. Zawodzinski, et al, Electrochem. Soc. Proc., Vol. 95-23 (1995) 57-65; A. K. Shukla, et al, J. App. Electrochem., 19(1989) 383-386; U.S. Pat. No. 5,702,755 to Messell; U.S. Pat. No. 5,859,416 to Mussell; U.S. Pat. No. 5,501,915 to Hards, et al.
To reduce dependency on the importation of oil, it has been suggested that the U.S. economy be based on hydrogen as opposed to hydrocarbons. The current atmosphere surrounding the hydrogen economy is supported in part by the success of the PEM fuel cell. As previously said, a primary cost relative to the manufacturer of PEM fuel cells is the noble metal, such as platinum, used as the catalytic electrodes. Importantly, the Nafion® membrane is also a relatively expensive material and contributes to the cost of the fuel cell stack. Typically, the average life of a fuel cell is about one year. Pinholes in the membrane and catalyst deactivation are some causes which reduce the effectiveness and, thus, useful life of the PEM fuel cell.
Recycling of the membrane electrode assembly, which typically contains a core of Nafion® membrane and the platinum/carbon electrodes coated on either side thereof, can address several of the cost issues related to manufacture and use of the PEM fuel cell. First, recovery of the platinum catalyst for reuse is important to meeting the world demand for the metal, and helping to maintain a reasonable price for the metal. Current commercial recovery of platinum from an MEA involves the combustion of the membranes and the processing of the ash. This mechanism is useful because it generates an ash that can be assayed for the purposes of commercial exchange. Unfortunately, there are two disadvantages with this prior process. First, ignition of the fluoropolymeric Nafion® membrane and the PTFE used often in the gas diffusion layers yields HF gas, which is corrosive and hazardous to health. Discharges of HF gas are highly regulated, and even with scrubbing of the gas, furnace throughput is constrained because of residual HF. Secondly, the burning of the Nafion® membrane destroys an expensive, value-added material.
Co-pending U.S. patent application No. 11/110,406 teaches that the Pt/carbon catalyst layers of a membrane electrode assembly can be recycled by contacting the MEA with a lower alkyl alcohol solvent. According to the '406 application lower alkyl alcohols disrupt the bond between the membrane and the attached Pt/carbon catalyst layers allowing for separation of the Pt catalyst layers from the intact membrane.
Methods for dispersing fluorocarbon-containing ionomer polymers are known, which may be adapted to recover the membrane, including those disclosed in U.S. Pat. Nos. 6,150,426 and 4,433,082. The '426 patent discloses a process for preparing a highly fluorinated ion-exchange polymer by dispersing the polymer under pressure in an aqueous liquid dispersion medium. According to the '426 patent the polymer can be dispersed in a medium consisting essentially of water, under pressure, at preferred temperatures of 150° C. to 350° C. In the '082 patent, a process is provided for making a liquid composition of a perfluorinated polymer by contacting the polymer with a mixture of 25 to 100% by weight of water and 0 to 75% by weight of a second liquid component such as a lower alcohol, e.g., propanol or methanol, at a temperature of at least 180° C. However, these methods tend to be cost prohibitive due to the high pressure and temperature requirements for dispersion of the fluropolymeric membrane.
Alternative processes have been proposed for MEA recycling. These processes do not address the issue of recycling to the extent of the present invention. For example, one process uses a fusion process to recover the precious metal from the MEA. The 3-layer MEA is processed in a flux containing calcium salt. This sequesters the liberated HF as CaF2. However, the value of the MEA membrane is destroyed. Another process dissolves the MEA membrane and proposes to recast the membrane film and re-use the recovered electrode catalysts. Experience has shown that the physical properties of the membrane change during aging. Recasting a film with lower molecular weight polymer may result in a membrane with different properties than one made with virgin polymer.
Accordingly, it would be useful to provide an alternative process for recycling the membrane electrode assembly of a PEM fuel cell whereby the noble metal is recovered in high yield and the Nafion® or other fluoropolymeric membrane is completely recovered for potential recycling. Such a process in which there are no serious environmental issues such as the formation of HF gas can be operated with low-energy utilization, and whereby the process facilitates a commercial exchange based on the assay of the recovered noble metal would aid in promoting the hydrogen economy.